1. Field of the Invention
This invention relates to a method for producing polyacrylic acid by utilizing steam and chilled liquid generated in an acrylic acid production process, and to an apparatus for implementing the same method.
2. Description of the Prior Art
Acrylic acid is a raw material for various chemical products. Particularly, there has been reported that desired polyacrylic acid such as water-absorbent resin is producible in a polyacrylic acid production plant built in the same site or in vicinity of an acrylic acid production plant, by using high-purity acrylic acid produced in the acrylic acid production plant as a raw material.
Acrylic acid is produced by various methods. For instance, one method is known in which propylene and/or propane is fed to an evaporator for gasification, and the gasified propylene and/or propane is fed to a gas-phase catalytic oxidation device where the gasified propylene and/or propane is contacted with a gas in the presence of a catalyst to yield reaction product containing acrylic acid and byproduct. Then, the reaction product and a absorbing solvent are fed to a absorbing device to yield acrylic acid aqueous solution containing the byproduct, which is then drawn to a distillation/purification device including azeotropic distillation column, heavy-ends cut column, and rectification column for distillation and purification in this order sequentially, thereby producing acrylic acid of high purity. Further, the thus obtained high-purity acrylic acid is fed to a polyacrylic acid production plant built at the same site or in vicinity of the acrylic acid production plant to produce desired polyacrylic acid such as water-absorbent resin.
Particularly, in producing high-purity acrylic acid of large quantity with inexpensive cost on an industrial scale, efforts have been made to reduce production cost by recycling various substances discharged from certain processes for reuse in other processes, in addition to regulating the operating conditions in the respective production processes. For instance, Japanese Unexamined Patent Publication No. 2001-131109 discloses a method of recycling heat medium used in gasifying liquefied propylene in an evaporator. In this publication, heat medium is transformed into liquid chilled coolant by utilizing latent heat generated by gasification of liquefied propylene or the like. In this technology, latent heat, which has been wasted conventionally, is utilized by using a heat exchanger such as cooler and condenser used in an acrylic acid production process so as to save energy for cooling.
In an acrylic acid production process, steam is obtained by recovering heat of reaction generated in a gas-phase catalytic oxidation device. However, in addition to the above-mentioned steam obtainable from the heat of reaction, steam is obtainable by various ways. For example, steam is recovered from a cooling step of cooling reaction product generated in a gas-phase catalytic oxidation device, or from a recovering step of recovering heat of combustion generated at the time of burning waste gas or waste liquid discharged from an acrylic acid production process.
Reuse of the steams recovered from the above different steps has been practiced in the conventional acrylic acid production process. However, the reuse has been insufficient. For instance, in the case where steam recovered by a heat recovering device such as a boiler for recovering heat of reaction generated in a gas-phase catalytic oxidation device is high-pressure steam having a pressure of not lower than 1.0 MPa gauge (the temperature is not specifically limited, but normally is not lower than 183° C. and lower than 270° C.), acrylic acid in the production process may be overheated if such high-pressure steam is used as heat medium for a heating device such as a reboiler which is attached to an acrylic acid distillation device, with the result that polymerized substance may be generated. Hereinafter, the gauge pressure is simply indicated as “G”. In view of such a drawback, high-pressure steam is required to be depressurized before use. Further, in the case where steam is medium-pressure steam of not lower than 0.6 MPaG and lower than 1.0 MPaG (temperature: e.g. not lower than 164° C. and lower than 183° C.), such medium-pressure steam is usable as heat medium for heating in an azeotropic distillation column or a high-boiling-component-removing column without depressurizing the steam. However, it is likely that polymerized substance may generate in a column at a final stage of producing high-purity acrylic acid even with supply of such medium-pressure steam. Accordingly, it is also necessary to depressurize medium-pressure steam for use at such a final stage of high-purity acrylic acid production, and the number of locations where medium-pressure steam is usable is limited. In view of the above, it is desirable to feed low-pressure steam as heat medium into a reboiler which is attached to a column for producing acrylic acid as final product, wherein the low-pressure steam is not lower than 0.2 MPaG and lower than 0.6 MPaG (temperature: e.g. not lower than 133° C. and lower than 164° C.). It is needless to say that these low-pressure steam, medium-pressure steam, and high-pressure steam are used as heat medium for a heat exchanger other than heating a distillation column, as well as for a power source for driving a turbine. However, consumption quantity of steam in an acrylic acid production process relative to the total quantity of steam derived by recovering heat of reaction and heat of combustion in the acrylic acid production process decreases, particularly, as the pressure of the steam to be supplied increases. This means that the higher the pressure is, the less the number of locations where the steam is usable as heat medium is. Consequently, the consumption quantity of steam is lowered compared to the generated quantity of steam, with the result that a large quantity of the steam is discharged outside of the reaction system without reuse. Particularly, since high-pressure steam is required to be depressurized before use, high-pressure steam is seldom reused and most of the high-pressure steam is discharged outside of the reaction system.
Japanese Unexamined Patent Publication No. 2001-131109 discloses a recycling technique of latent heat (liquid chilled coolant) as to how to reuse latent heat recovered by gasification of liquefied propylene in an evaporator. However, an integrated recycling system has not been developed so far, wherein a polyacrylic acid production process for producing polyacrylic acid by feeding acrylic acid produced in an acrylic acid production process to the polyacrylic acid production process, and the acrylic acid production process are integrated.
In view of the above, an object of this invention is to provide a method for efficiently utilizing steam obtained by recovering heat source such as heat of reaction generated in an acrylic acid production process, in a combined process of an acrylic acid production process and a polyacrylic acid production process for producing polyacrylic acid by using acrylic acid produced in the acrylic acid production process, and a method for efficiently utilizing latent heat recovered by gasification of propylene and the like.
Another object of this invention is to provide an apparatus capable of implementing the above method.